An electrophotographic photoreceptor is the component which provides an imagewise latent image consisting of electric charge on the surface thereof when it is irradiated along an image to be developed. An electrophotographic photoreceptor therefore has a charge generating layer (CGL) which generates charge by irradiation on a conductive substrate.
An electrophotographic photoreceptor have been widely applied to an electrophotographic apparatus such as a copying machine, a printer, and the like. In recent years, semiconductor laser ray, which is a compact and low-cost light source, is mainly employed as a light source for an electrophotographic photoreceptor of such an electrophotographic apparatus. Thus, an organic photoconductive substance (OPC) which is sensitive to emission wavelength of a semiconductor laser, has been a matter of interest in the art.
OPC is the material which comprises a bonding or dispersing agent consisting of an organic resin (binder), and a charge generating material (CGM) having photoconductive property dispersed in the binder. An electrophotographic photoreceptor is prepared, for example by applying OPC layer on an photoconductive substrate.
This type of electrophotographic photoreceptor which is equipped with the CGL made by OPC is generally referred to as an organic photoreceptor. As CGM, a functional pigment having photoconductive property such as a stable or a semistable phthalocyanine compound is generally employed.
A phthalocyanine compound has high spectral sensitivity through long wavelength region, high charge generating efficiency, high sensitivity, and high durability. Thus, a metal or a metal free phthalocyanine compound, such as x-form metal free phthalocyanine, titanyl phthalocyanine, and vanadyl phthalocyanine, has been employed in an organic photoreceptor.
An electrophotographic photoreceptor may be further equipped with CTL on a surface of CGL so that the charge occurred in CGL arrives efficiently at a surface of the photoreceptor, or so that the photoreceptor has sufficient strength. CTL is generally composed of a binder consisting of an organic resin, and a charge transporting material (CTM) dispersed in, or dissolved in the binder.
As CTM, a polyvinyl carbazole resin, an azole, a pyrazoline, a hydrazone, an enamine, a styryl, a stilbene, a triphenylmethane, a triphenylamine, a dibenzylaniline, and an azine compounds are known in the art.
In recent years, a semiconductor laser ray is employed as a light source of a copying machine or a laser printer, and as an electrophotographic photoreceptor for such a light source, a layered-form electrophotographic photoreceptor is employed. The wording "layered-form electrophotographic photoreceptor" means the electrophotographic photoreceptor which has a bi-layered photoreceptor layer, that is a photoreceptor layer composed of CGL and CTL. More currently, use of the layered-form electrophotographic photoreceptor covers over a printer having a short wavelength LD ray or light emitting diode as a light source.
In the art of electrophotographic photoreceptor, the monolayered-form electrophotographic photoreceptor which has a photoreceptor layer having a combination of amorphous selenium and polyvinylcarbazole is well known. Japanese Patent Kokai Publication 163047/1985 for example discloses an improved organic photoreceptor of this type, which contains CGM, CTM, and sensitizing dyes in a photoreceptor layer.
As a layered-form photoreceptor, Japanese Patent Kokoku Publication No. 41021/1990 discloses those having a photoreceptor layer composed of a selenium layer, and a polycarbonate layer containing a hydrazone CTM laid on the selenium layer; Japanese Patent Kokai Publications No. 196767/1985, and 262162/1985 disclose those having a photoreceptor layer composed of CGL containing a disazo CGM and a hydrazone CTM, and CTL comprising a polycarbonate resin and a hydrazone CTM laid on the CGL; Japanese Patent Kokai Publications No. 48551/1988, 48552/1988, and 4260/1992 disclose those having a photoreceptor layer composed of CGL containing a phthalocyanine pigment, and CTL containing a specific (bis)hydrazone CTM laid on the CGL; and Japanese Patent Kokai Publication No. 233548/1992 discloses those having a photoreceptor layer comprising a benzothiazole CTM.
However, the abovedescribed organic receptors are insufficient in electric properties (electrophotographic properties) such as chargeability, dark decay ratio, and residual potential, and in durability when they are charged and exposed many times. Partially azoic compound is now known as CGM responsible to a short wavelength light source such as LED and 633 LD, but it is poor in light resistance and durability by comparison with a phthalocyanine compound. Therefore there is a need for a photoreceptor having sufficient durability even if they are charged and exposed many times, and having sufficient sensitivity to a short wavelength light source.
The inventors disclosed in Japanese Patent Application No. 25206/1996 that .mu.-oxo-aluminum phthalocyanine dimer having a novel polymorph was suitable as CGM for an organic photoreceptor having middle to high sensitivity. Some layered-form electrophotographic photoreceptors by using the .mu.-oxo-aluminum phthalocyanine dimer as CGM in combination with some commercially available CTMs such as a hydrazone derivative and a butadiene derivative, are prepared and evaluated in the application. As a result, it was discovered that II-form polymorph of the .mu.-oxo-aluminum phthalocyanine dimer was particularly excellent in electrophotographic properties as CGM. The inventors further discovered that the II-form polymorph had specific spectral sensitivity in the short wavelength region of 600 to 650 nm.
The inventors further tried based on such a background, many combinations of the II-form polymorph and various CTM, and found that the photoreceptor layer composed of CGL containing the II-form polymorph as CGM, and CTL containing a specific hydrazone CTM having a benzothiazolidene moiety laid on the CGL, shows remarkable electrophotographic properties.